Call me old-fashioned, but when I think "animated musical," I tend to associate it with original music. Songs designed to fit a specific story, and which are henceforth associated with that story. There's such a rich history behind it that it doesn't seem like too much to ask for. Perhaps it's just a symptom of being raised on Disney movies as a kid, but even so, Disney seems to have been doing something right - and plenty of other films took the lead using the same synthesis of cinematic and musical ideas. In other words, animated musicals were designed like ... y'know, musicals.

And yet ... I mean, it may be premature to call this a "trend," exactly, but on the heels of last year's repulsive Strange Magic, and a month away from the American Idol-flavored Sing, sits the latest offering from DreamWorks Animation, Trolls, which is feature-length karaoke in place of an actual musical. Its soundtrack - sung, to the film's credit, by a voice cast with proven chops, among them Anna Kendrick, Zooey Deschanel and Justin Timberlake - is made up largely of refashioned versions of pop songs. (Because as we can all agree, the way we really want to hear the music of Earth, Wind & Fire, Simon & Garfunkel and The Notorious B.I.G. is in chintzy medley versions that sound like product jingles.)

This certainly isn't the first karaoke musical and it's certainly not the worst, either; but the choice to go with synthetic-sounding pop covers gives the whole thing a queasily vulgar sensibility. To be fair, there are some original songs as well - from the likes Ariana Grande and Justin Timberlake, who also arranged and produced much of the rest of the music in what he surely must consider a career highlight, and exactly the type of break a young kid like him needs. But while those songs - including Timberlake's summer hit "Can't Stop the Feeling!" - are catchy earworms, they're hardly memorable.

Worse than that, it smells more than a little like cynical cross-promotion rather than a union of material and collaborative artist. DreamWorks gets to use Timberlake as a selling point, and Timberlake gets to expand his appeal to the millions of kids who will see Trolls - and, even worse, convince their parents to download the soundtrack for them - now and for years afterward. Sounds like a win-win. Except for everyone who likes good musicals. Bottom line: It's usually a bad sign for a movie when the trailers and TV spots prominently emphasize the artists on the soundtrack as much as the film itself.

And that's all, of course, on top of the cynical nature of the film's entire existence, based as it is on a line of plastic dolls that have been licensed for various purposes over the years. DreamWorks actually owns the entire brand, making it a sort of self-contained money machine that they can exploit every few years when they need a few hundred million dollars. No doubt the studio's talented marketing department storytellers will find various new and exciting ways to reimagine and re-brand the Troll dolls, including but not limited to a line of inevitable sequels. Maybe Justin Bieber can do the soundtrack next time.

Despite an idea that, on paper, is a nightmare, you want to hope for the best for something like Trolls. You want to hope that it turns out like The Lego Movie. Your gut tells you it's more likely going to be closer to the Transformers or G.I. Joe movies ... or worse yet, like direct-to-video movies based on Barbie or Hot Wheels or My Little Pony. But you think and hope ... maybe some weird writer or two can find a way to turn this into something unexpected. And to the film's credit, some of its creators are exactly the type. Both the two screenwriters - Jonathan Aibel and Glenn Berger - and director Mike Mitchell were collectively responsible for last year's deliriously absurd The SpongeBob Movie: Sponge Out of Water, which not only had sequences of brilliance but generally ran on an entirely different wavelength than standard CG animated cinema.

The ultimately reality is that Trolls is somewhat more than we might fear, and much less than we might hope. There are bursts of twisted inspiration here and there, but ultimately the film seems unwilling to follow them too far. They make for good moments in an otherwise unsatisfactory film. Convention basically rules the day, as we get a standard-issue tale of love conquering all, underdogs escaping monsters, cold hearts turning warm. More than anything, Trolls is an ode to happiness, with its title characters - basically multi-colored Smurfs - being envisioned as the happiest creatures on Earth. They sing and dance all day and give each other group hugs every half-hour, on the dot. The only bad thing that happens is ... well, every year, the odious Bergens (monstrously unpleasant creatures who have their own kingdom overseeing the Trolls' little forested world) celebrate a holiday called Trollstice, in which they eat the poor Trolls, as it is the one thing in the world that can give the generally joyless Bergens a taste of happiness. (This was someone's excuse for a plot idea, so let's just go with it, OK?) At the beginning of the film, they secretly escape - depriving the Bergens of their favorite (only?) holiday - and live the next decade or so in peace and happiness.

In fact, we can always tell exactly how happy and contented the Trolls are, simply by the color of their fur; the bright colors aren't just simply a design choice, but an external manifestation of their joy. Pink must be, in the film's mind, the most joyful color, as its most joyful character is Princess Poppy (Kendrick). She spends most of the movie on a mission to save her fellow Trolls from the clutches of the nasty Bergen Chef (Christine Baranski) - who has been on the hunt for those delicious, sneaky Trolls ever since their departure cost her her livelihood - while simultaneously trying to convince the sad-sack survivalist troll Branch (Timberlake) to let loose and sing and dance and be happy like everyone else. (If you're wondering whether there is a tragic backstory that explains why Branch is so sad, and whether he will finally lighten up by the end of the movie, well you'll just have to keep on guessing. I'll never tell.)

Where the film does stand out is in the detail of its animation. The animators are clearly going for more texture and physicality than in most computer animation, with the characters' felt-like bodies looking like objects you could pick up. They certainly looks much more pleasant than the mass-marketed plastic versions that I remember growing up. In terms of character design, there's only so much the movie can do - the Trolls kinda have a specific look - but with the Bergens, at least, it seems the filmmakers have taken inspiration from the right place, that being Laika's The Boxtrolls. There's a distinct similarity in the Bergens' gnarled, warty, angular and squared-off features to Laika's creations from a few years ago, with Chef looking like a bastard child of the villainous Archibald Snatcher and an actual Boxtroll.

Still, whatever its merits, Trolls seems - like most DreamWorks movies - content to play it right down the middle. I don't know if a more distinctive, original soundtrack would have made it all worthwhile, but I least would have liked to see - and hear - that version of the movie over the one we got instead.